Transducers are used to provide electrical signals corresponding to the value of a physical phenomenon such as fluid pressure or the strain in a structure. Calibration of a monitor for displaying the value of the phenomenon can be effected by setting its display to zero when the value of the phenomenon is zero and by setting its display to a given value of the phenomenon when that value is applied to the transducer. In blood pressure monitoring apparatus for example a zero setting can be made while the transducer is exposed to atmospheric pressure, and a gain setting can be made when some known pressure such as a 100 mm of Hg is applied to the transducer. As the latter is a cumbersome procedure some monitors are equipped with calibration circuits that add, to the output of a transducer when it is exposed to atmospheric pressure a voltage corresponding to a pressure that is determined by known transducer sensitivity of 5 uv/Ve/mm Hg. Ve is the operation voltage across one diagonal of a bridge circuit in the transducer that is imbalanced in accordance with the blood pressure so as to produce a fraction of Ve as the transducer output signal. Unfortunately, however, many monitors now in use do not have calibration circuits so that it is necessary to use the cumbersome procedure of applying a known pressure to the transducer.